The therapeutic use of oligonucleotides is a field of great significance and is described, for example, in, (1) Zamecnik, P. C. and Stephenson, M. L. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 280, 285."; (2) Uhlmann, E. and Peyman, A. Chemical Reviews, 1990, 90, 543-584; (3) Goodchild, J. Bioconjugate chemistry, 1990, 1, 165-187; and (4) Crooke, S. T. and Lebleu, B. "Antisense Research and Applications", CRC Press (1993)). The specific binding of antisense polynucleotides to the DNA or RNA targets of interest may inactivate the functions associated with the DNA or RNA such as replication, transcription, or translation, thereby providing a mechanism for controlling diseases such as cancer and viral infection. Therefore, the binding of an antisense oligonucleotide to a target can be used to alter gene expression, in a variety of circumstances, e.g., to interfere with viral life cycles, or the growth of cancerous cells (Stein, C. A., Cheng, Y. C. Science, 1993, 261, 1004-1012). In addition, some oligonucleotides also bind tightly to protein targets, thereby acting as enzyme inhibitors. Bock et al. describes oligonucleotides that inhibit human thrombin-catalyzed fibrin-clot formation in vitro (Bock, L. C., Griffin, L. C., Latham, J. A., Vermaas, E. H., Toole, J. J. Nature, 1992, 355, 564-566). Ecker et al describes several oligonucleotides that inhibit human herpes simplex virus at below 1.0 .mu.mol. Polynucleotides that have enzyme inhibiting properties can readily be found by using combinatorial technology (Ecker, D. J., Vickers, T. A., Hanecak, R., Driver, V., Anderson, K. Nucleic Acids Res. 1993, 21, 1853-1856).
An oligonucleotide containing a 5'-C-methyl branched nucleoside has been reported to show enhanced nuclease resistance (Saha, A. K. et al., a poster in 206th ACS Meeting, Chicago, 1993). An oligonucleotide containing 2'-O-methyl nucleosides has also been reported to show improved stability to nucleases and enhanced binding affinity to RNA (a. Inoue, H., Hayase, Y., Imura, A., Iwai, S., Miura, K., Ohtsuka, E., Nucleic Acids Res. 1987, 15, 6131; b. Shibahara, S., Mukai, S., Morisawa, H., Nakashima, H., Cobayashi, S., Yamamoto, N. Nucleic Acids Res. 1989, 17, 239). An oligonucleotide containing 1'-substituted nucleoside has been reported to show some nuclease resistance (Ono, A., Dan, A., Matsuda, A. Bioconjugate Chemistry, 1993, 4, 499-508).
Besides having a specific binding affinity to a complementary target polynucleotide sequence, antisense oligonucleotides desirably meet the requirements for therapeutic purposes, e.g., potency, bioavailability, low toxicity, and low cost. Since oligonucleotides having the natural phosphodiester backbone are labile to nucleases and do not readily penetrate the cell membrane, researchers have attempted to make polynucleotide backbone modifications that improve nuclease resistance and cellular uptake. A major shortcoming of oligonucleotides analogs used for antisense is that the modified internucleotide linkages eliminate the RNase H activation of antisense oligonucleotides, which degrades the RNA strand to which the oligonucleotide analog binds. Therefore, it is desirable to provide polynucleotide analogs with enhanced nuclease resistance and cellular uptake, while retaining the property of activating RNase H.